The present application relates to a dynamic vascular compliance tester apparatus for determining vascular compliance of a vascular test specimen or an artificial graft for a vascular implant.
One of the most controversial areas in the field of vascular grafts has been the concept of compliance matching. The elastic properties or viscoelastic properties of tubular elements have been of interest for many years, particularly in the area of cardiovascular research. Medical research indicates that, when arteries and veins get stiff or lose their elasticity, they become very susceptible to atherosclerosis. In addition, if a compliance matching problem arises after the implantation of blood carrying vessels including artery or vein sections (both when removed from another portion of the body or when artificial), blood will sometimes deposit coagulation factors at the interface between the original vessel and the implanted vessel.
By determining the change in diameter or radius of the vessel and by simultaneously observing the pressure of the pulse which creates the change in radius of the vessel, a quantity commonly referred to as compliance can be calculated. For this type of testing, compliance is referred to as the percent change in radius which occurs when the corresponding blood pressure changes by the equivalent of 100 mm Hg pressure. (In actuality, blood pressure changes are somewhat less than 100 mm Hg but the results are extrapolated to provide an equivalent change in radius for the 100 mm Hg change in pressure for comparison purposes.)
During the past several years, various high-tech methods have been utilized in an attempt to measure the mechanical properties of arteries in vivo or in the body. The variety of methods which have been utilized are so fraught with complications and errors that those methods are almost useless. For example, one common technique used with arteries is to expose the vessel by surgically removing the surrounding tissue. A lever, which is connected to an electronic measuring instrument, is then placed against the outside wall of the artery. As the artery pulses, the lever is moved in proportion to the expansion and a measurement of the displacement of the lever is recorded. This technique has several flaws associated with it. In particular, when an artery is totally exposed to the air, it begins to constrict. Such a constricted vessel has lower elastic properties than those of a natural vessel. Further, the point where the lever touches the vessel becomes traumatized and does not respond to the varying blood pressure in the same manner that the remainder of the artery responds. As a result, erroneous data is obtained from this type of testing.
Another technique which has been attempted is an ultrasonic method whereby ultrasonic producing piezoelectric crystals are attached to the vessel. This approach has many of the same problems as those associated with the aforedescribed lever technique such as vessel exposure. In addition, the magnitude of the ultrasonic signal is functionally dependent upon the various angles involved with the topology of the sensor and the artery. As a result, any slight change in the critical angles can cause discrepancies in the resulting data obtained. Further, it is almost impossible to perfectly align the source of the ultrasound with the crystal because the position of the crystal usually shifts, however minutely, after surgical implantation thereof.
Another weakness of both of the foregoing techniques is that only the outside diameter of the vessel is measured; whereas, the dimension of the vessel which is of critical importance is the inside diameter. In addition to measuring the physical properties of actual vessels, it is desirable and beneficial to have a simple method of measuring the physical properties of synthetic vessels currently being developed.
It is also important to recognize that the physical properties of viscoelastic vessels normally change as the pulse frequency at which vessels are being tested changes. Specifically, many laboratory procedures utilize static techniques to measure the physical properties of tubing or vessels. When the same vessels are then used in situations where they are subjected to dynamic pressure changes, such as in a functioning artery, then the dynamic properties of those vessels may vary substantially from the measured static properties. The dynamic properties, which are the more important ones, may differ from those anticipated using static data.